Electric cable



Dec. 21, 19,37. D, M, RQBINSON 2,102,974

ELECTRIC CABLE Filed June l2, 1935 Patented Dec. 2l, 1937 UNITED STATESaioaem ELECTRIC `CABLE Denis Morrell Robinson, London, England, as-

signor to Callenders Cable and Construction Company Limited, London,England, a British Application June 12, 1935, Serial No. 26,135 In GreatBritainJune 22, 1934 2 Claims.

The invention relates to high tension electric cables of the kind inwhich the dielectric is formed of laminated and impregnated material,usually paper impregnated with oil or compound 5 (hereinafter forconvenience referred to as compound). Cables of this type, whether thesolid part of the dielectric consists of paper or other fibrousmaterial, will hereinafter be termed, for convenience, impregnated paperinsulated cables.

In cables of this kind there may be present in the part of the cablesubjected to electrical stress, small gaseous or vacuous spaces, usuallytermed voids. These voids may be due `to incomplete impregnation of thebody of insulation but more usually they are caused by subsequentmigration of the impregnating medium in a radial and/or longitudinaldirection due, for instance, to the thermal cycle which the cableundergoes during its working life or to drainage.

It has generally been assumed that such voids usually constitute theinitial cause of breakdown of the cable. Various proposals have beenmade for inhibiting, or reducing the probability of, the formation ofsuch voids.

been proposed to prevent the production of voids during the cooling partof a cycle by facilitating return fiow of the impregnating medium intothe body of fibrous dielectric by means of oil ducts and/or maintainingthe interior of the cable under a pressure greater than atmospheric.Cables in which return flow of the compound is so facilitated haveproved to be satisfactory from an electrical point of view butunfortunately necessitate the use in conjunction therewith of expensiveauxiliary plant. In British Patent No. 175,753, it has been proposed toprevent migration by enclosing the insulation by means of thinseparating layers of a non-metallic material which is impervious to thecompound with which the brous dielectric isI impregnated. One of theselayers encloses the conductor in order to prevent the flow of compoundinto the interstices `of the conductor and the production of voidsduring the cooling part of a cycle due to the disinclination of thecompound to flow back from the interstices into the brous insulation.These precautions may result in the inhibition of the production ofvoids within the dielectric body, but this inhibition is not, forlreasons to' 5 0 be explained later, sufiicient to make the cableresistant to breakdown. In British Patent No..

For instance, it has favour of a dielectric consisting wholly of a waterresistant thermoplastic compound applied in layers by extrusion.Insulation of this kind is at present relatively expensive as comparedwith impregnated paper insulation. Moreover 5 such a cable would notpossess the degree of iiexibility possessed by a paper insulated cabledue toits insulation being built up of successive layers that are freeto slide over one another and are formed of helical lappings of tape. l0

- The problem before the manufacturer of super-tension cable istherefore to produce an impregnated paper insulated cable which isresistant to breakdown under Working conditions but does not necessitatethe use in conjunction l5 therewith of expensive auxiliary plant. Withthis in View the applicant has conducted extensive researches as aresult of which he discovered that the breakdown of the dielectric ofimpregnated paper insulated cables is in the 20 great majority of casesinitiated at or close to the conductor in spaces containing gas andhaving the conductor as the inner boundary and the dielectric as theouter boundary.

yThe starting of the process which results in 25 breakdown is usuallydue to the bombardment of the outer boundary surface of such a space byions liberated from the gas in the space and propelled by the electricforce in the space. This bombardment causes carbonization either of the30 compound or of the paper. It is found that the carbonization of thecompound can proceed within and through the paper, owing to the factthat paper is not homogeneous. The paper is made up of fibres feltedtogether and containing 35 or enclosing'cells or passages which are madeto contain compound as the result of the impregnating process.Carbonization can proceed along threads of compound extending throughthese passages. v

In the light of this discovery it appears, contrary to the opinionhitherto held by cable manufacturers, that the object to be aimed at isthe elimination of possible gas spaces bounded by the surface of theconductor rather than the elimi- 45 nation of possible gas spaces withinthe body of insulation constituting the dielectric. This statement doesnot imply that the presence of voids within the insulation is notundesirable but that the existence of such voids is notY so conducive tobreakdown as is that of voids between the surface of the conductor andthe insulation.

These researches suggest that'the problem set out above may be solved byinterposing between the surface of the conductor and the innermost layerof the impregnated body of dielectric, a thin but continuous layer thatis in complete contact with the conductor throughout the inner surfaceof the layer and is formed of a homogeneous insulating materialimpervious to oil or compound.

For the purposes of this specification the term homogeneous insulatingmaterial implies an insulating material which is not built up of fibresthat retain their individuality and which does not contain small voids,cracks or microscopic channels such as are present in paper.

Materials which are regarded as suflciently homogeneous for the presentpurpose and are mentioned by way of example are found amongst thefollowing:-esterified cellulose compositions,

'such as those having a cellulose acetate basis;

cellulose ethers such as benzyl cellulose; polystyrenes, for instancestyrol; and oil-resisting rubber-like compounds, for instance, the olenepolysulphide compounds sold under the trade name Thiokol.

From my researches it appears that the continuity of the covering mustbe such that there is no passage in a radial direction through thelayer, but I have found that it is permissible to build up the layer ofthin helical strips of material applied with a. substantial overlap,providing that where gaps exist between the adjacent surfaces of theconductor and of the layer these vare reduced to a very small value, saynot exceeding 2 mils (0.05 mm.) in-radial measurement.

The present invention accordingly comprises a novel construction ofpaper insulated cable embodying in a practical manner the results of myresearches. The improved 'cable comprises a conductor of which thesurface is made smooth and of non-reentrant configuration and,interposed between the conductor and the innermost layer of thesurrounding body of impregnated paper insulation, a comparatively thin(that is thin in comparison with the surrounding body of insulation)continuous layer, of homogeneous insulating material impervious to oilor compound. This layer is formed by helically lapping the conductorwith one or more strips of such material, the thickness of the stripsbeing not more than 2 mils (0.05 mm.). The use of such a conductor incombination with the impermeable layer built up of strip of thesedimensions ensures that gaps between the adjacent surfaces oftheconductor and of the layer do not exceed 2 mils in radial thickness.

In order to provide a margin of safety it will generally be advisable toform the layer of homogeneous material of several sup'erposed layers ofstrip. For instance where each strip is applied with a small gap betweenits successiveturns. lsix strips may be used to build the homogeneousayer.

Smoothing the surface of the conductor may be effected by building it,or the outer part of it, from wires of a section appropriate to give acontinuous cylindrical form. Alternatively, a strand formed of roundwires may be covered by a sheath of metal, for instance of lead,extruded over it, or by a metal tape or a metallized paper tape lappedtightly over it so as to cover the entire surface. In another form ofconstruction, the innermost strip of the layer may have a metallizedinner surface and be applied with a substantial overlap.v The metallizedsurface of the strip makes contact with the conductor and therebyextends the surface of the latter t9 the place where the metallizedlayer joins the homogeneous material on which it is carried.

In the accompanying drawing, which shows an example of construction ofhigh tension impregnated'paper insulated cable according to theinventlion,

Figure 1 is a cross-sectional View of the cable, and

Figure 2 is a fragmental sectional elevation, on a greatly enlargedscale in a radial direction, taken on the line H-II in Figure 1, of thecable shown therein. i

In the construction of the c able shown in Figures 1 and 2, theconductor II, consisting of a plurality of wire strands I2, is insulatedby a thick body of impregnated paper insulation I3 and enclosed within alead sheath I4. A smooth surface of non-reentrant configuration isimparted to the conductor by covering it with a continuous sheath ofmetal I8 in contact with some or all of the outer wires of theconductor. On this smooth surface is applied the layer I5 of homogeneousinsulating materiali This layer is built up of overlapping turns ofstrip I9 of which the thickness is less than 2 mils (0.05 mm.)

'I'hese strips I9 may be 1 inch wide and applied with a gap of 11g inchbetween adjacent turns of the same layer. Each turn of a succeedinglayer is arranged to overlap by a substantial amount two adjacent turnsof the underlying layer. The overlap may, for instance, be 50:50, asshown in the case of the rst and second layers or 75:25 as shown in caseof the second and third layers. It will be appreciated that for the sakeof clearness in Figure 2 the tapes are shown spacedA apart in a radialdirection and that they and the sheath I8 are drawn to a scale in aradial direction which is 60 to '70 times the scale in the longitudinaldirection.

The outer part of the dielectric, thatgis to say the part beyond thelayer covering the conductor, consisting of impregnating paper orequivalent material, may be applied and treated in the usual' and thismay be carried to such a point that the eiect, which previously wasconned to the conductor, is made evident also at the sheath. In suchconditions, steps analogous to those set out in the-precedingdescription can be applied in accordance with the present invention tothe region between the outer part of the dielectric and the sheath.4 Itmight, for instance, be carried out as shown in the drawing by applying,between the usual metallized layer I1 on the outside of the dielectricand the dielectric, a covering I6 which is of the same kind andthas thesame relation to the metallized layer as the covering hereinbeforedescribed in connection with the conductor.

What I claim as my invention isz- 1. An impregnated paper insulatedcable comprising a conductor having a smooth surface of non-reentrantconguration, a comparatively thin continuous covering of homogeneousinsulating material impervious to oil and compound, surrounding saidconductor and in contact therewith, said covering comprising at least Vone lapping of strip, the thickness of which strip is not greater thantwo mils, an impregnated body of paper insulation surrounding saidcovering, and an impervious sheath enclosing the said body of paperinsulation.

2. An impregnated paper insulated cable comprising a conductor having asmooth surface of non-reentrant conguration, a comparatively lthincontinuous covering of homogeneous insulating material impervious to oiland compound surrounding said conductor and in contact therewith, saidcovering comprising a plurality of lappings of strip-like material, thethickness of the lapping of strip-like material being not greater thantwo mils, the adjacent edges of said lappings being spaced apart apredetermined distance and each layer substantially overlying itsunderlying layer, an impregnating body of paper insulation surroundingsaid covering, and an impervious sheathing enclosing the said body ofpaper insulation.

DENIS MORRELL ROBINSON.

